Derailleur for a bicycle



FIG. 1 is a top, right, front perspective view of a derailleur for a bicycle showing my new design;

FIG. 2 is a top, left, front perspective view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a bottom view thereof. 

The ornamental design for a derailleur for a bicycle, as shown and described. 